Generally, in air conditioning apparatus, thermal control is accomplished by intermittent operation of the compressor in response to a signal from a thermostat located in the room being cooled. Once the temperature in the room has been lowered to a desired temperature, the refrigerant capacity of the air conditioning system generally need not be very large in order to handle supplemental cooling because of further temperature changes in the room or for keeping the room at the desired temperature. Accordingly, after the room has cooled downs to the desired temperature, the most common technique for controlling the output of the compressor is by intermittent operation of the compressor. However, this intermittent operation of the compressor results in the intermittent application of a relatively large load to the driving mechanism of the compressor in order to drive the compressor.
In an automobile air conditioning compressor, the compressor is driven by the engine of the automobile through an electromagnetic clutch. Automobile air conditioning compressors face the same intermittent load problems described above once the passenger compartment reaches a desired temperature. Control of the compressor normally is accomplished by intermittent operation of the compressor through the electromagnetic clutch which couples the automobile engine to the compressor. Thus, the relatively large load which is required to drive the compressor is intermittently applied to the automobile engine.
Furthermore, since the compressor of an automobile air conditioner is driven by the engine of the automobile, the rotation frequency of the drive mechanism changes from moment to moment, which causes the refrigerant capacity to change in proportion to the rotation frequency of the engine. Since the capacity of the evaporator and the condenser of the air conditioner does not change, when the compressor is driven at high rotation frequency, the compressor performs useless work. To avoid performing useless work, prior art automobile air conditioning compressors often are controlled by intermittent operation of the magnetic clutch. However, this again results in a large load being intermittently applied to the automobile engine.
One solution to above-mentioned problems is to control the capacity of the compressor in response to refrigeration requirements. One construction to adjust the capacity of a compressor, particularly a wobble plate type compressor, is disclosed in the U.S. Pat. No. 3,861,829 issued to Roberts et al. Roberts et al. discloses a wobble plate type compressor which has a cam rotor driving device to drive a plurality of pistons and varies the slant angle of the slant surface to change the stroke length of the pistons. since the stroke length of pistons within cylinders is directly responsive to the slant angle of the slant surface, the displacement of compressor is easily adjusted by changing the slant angle.
In these prior art wobble plate type compressors with capacity adjusting mechanisms, the compressor should be provided with a rotation preventing device for the wobble plate. One rotation preventing mechanism is disclosed in the U.S. Pat. No. Re. 27,844 issued to Olson. The rotation preventing mechanism disclosed in the Olson patent comprises a pair of bevel gears, one of which is fixed on the center of the wobble plate and another which is supported on the housing, and a ball element seated in seating portion formed on the center portion of each bevel gear. Therefore, the wobble plate is supported on the ball element and rotation of wobble plate is prevented by engagement of bevel gears while permitting nutational motion along the ball surface. However, since in the above-mentioned wobble plate type compressor with a capacity adjusting mechanism, the slant angle of the wobble plate could be changed in response to the refrigeration requirements, the pair of bevel gears cannot be used as the rotation preventing mechanism.
Therefore, generally a pin rod connection is used in the wobble plate compressor as the rotation preventing mechanism. In this mechanism, the wobble plate is supported on the ball element or drive shaft but rotation of the wobble plate is prevented by a pin fixed to the lower end of the wobble plate. The pin is slidably fitted in an axial groove formed in an inner wall of the compressor housing. In this arrangement, the pin reciprocates along the groove with considerable sliding friction between the pin and groove thereby introducing a power loss. Particularly since sliding friction changes in response to the moving range of the wobble plate, the wobbling angular velocity which should be transferred to the wobble plate is not uniform during the changing of the slant angle of the wobble plate. Furthermore, the pin is subjected to an undesired force so that the reliability of the pin and the compressor unit as a whole is thus degraded, and the compressor housing is required to be large because the groove must be formed therein.